Polycarbonates are resins prepared by condensation-polymerization of an aromatic diol such as bisphenol A with a carbonate precursor such as a phosgene, and have excellent impact strength, dimensional stability, heat resistance and transparency. Thus, they are applied in a wide range of fields such as exterior materials of electrical and electronic products, automobile parts, building materials, and optical components. The method for preparing polycarbonates can be divided into a melt polymerization process and a solid phase polymerization process without using a phosgene, and an interfacial polymerization process using a phosgene.
First, the melt polymerization process is a process of performing polymerization in a state where the raw monomer is melted, and has the advantage of having a low risk because it does not use toxic materials, but high-temperature and high-vacuum facilities are needed during the treatment of highly viscous reactants, and thus there is a problem that the quality is deteriorated. The solid phase polymerization process is a process of performing polymerization at a temperature lower than the melting temperature after the crystallization of low molecular weight polycarbonate prepolymers. In the solid phase polymerization process, since toxic materials are not used and the reaction is performed in a solid state, it can suppress the deterioration of quality. However, there is a disadvantage that it requires a high-pressure equipment and it is difficult to apply to a continuous process.
As disclosed in U.S. Pat. No. 3,799,953, the interfacial polymerization process is a process in which an aromatic hydroxy compound such as bisphenol A and a gaseous phosgene are mixed in an organic solvent to perform a polymerization reaction in the interface between the aqueous solution layer and the organic solvent layer. This process has an advantage that polycarbonate can be relatively easily produced by a continuous process, but a step of removing the organic solvent is necessary.
Since the polycarbonates produced by the interfacial polymerization process as described above are dissolved in an organic solvent, a process of removing the solvent to obtain polymer particles is required, and this process is generally referred to as a solidification process.
In this regard, U.S. Pat. Nos. 4,546,172 and 6,214,892 disclose a method for obtaining water-dispersible polymer particles by spraying a solution in which a polycarbonate is dissolved onto water which is maintained at a temperature higher than the boiling point of a solvent, followed by vaporizing the solvent. However, in the case of the above method, since the solvent evaporates rapidly, the rate of particle formation is fast, but there is a disadvantage in that not only a large sticky agglomerate of particles can be produced but also an additional drying process is required to remove water present in the pores in the porous particles and a lot of energy is required for such a drying process.
Further, U.S. Pat. Nos. 4,668,768 and 7,947,803 disclose a method of using an anti-solvent as a method for obtaining polymer particles from a polymer solution. In the method, the polymer particles are obtained by using an anti-solvent which is thoroughly mixed with a solvent but does not dissolve polymers. However, there exists problems that an excess amount of the anti-solvent must be used as compared to the polymer solution, and that an additional process is required to isolate the anti-solvent from the solvent.
Recently, instead of the method described above, a method of removing a solvent from a solution in which a polycarbonate is dissolved by using high-temperature and high-humidity steam of a steam ejector is used. The method has the advantages that the size of the steam ejector can be increased and the installation is relatively simple.
However, since the steam is a high temperature, discoloration and hydrolysis of the polycarbonate occur in the process of removing the solvent. If discoloration occurs, it may be difficult to recover the polycarbonate in a subsequent process, and further the hydrolysis results in a decrease in molecular weight and a decrease in the physical properties of the polycarbonate. Thus, there is a need for a method that does not impair the physical properties of polycarbonate while removing a solvent from a solution in which a polycarbonate is dissolved using steam.
Accordingly, the present inventors have conducted an extensive research to develop a method of removing a solvent from a polycarbonate polymerization solution, and found that, when an antioxidant and a hydrolysis-resistant agent are added to a polycarbonate polymerization solution and then a solvent is removed using steam, the solvent can be effectively removed without impairing the physical properties of the polycarbonate, thereby completing the present invention.